1. Field of the Invention
The present invention relates to valves, and more particularly to gate valves used in the control of flow of materials. The invention is particularly applicable to gate valves which are utilized in handling bulk solids in installations where the pressure exceeds atmospheric.
2. Description of Related Art
Various valves are used in manufacturing and production facilities to control the flow of materials within the facility, or within systems in the facility. Slide-gate or knife-gate valves are commonly used to control a variety of material types, including bulk solids, liquids, gases, and slurries. These gate valves commonly have a structural frame with an opening for passing material with a sliding blade to block and unblock the opening. The blade is typically driven by a linear actuator using pneumatic or hydraulic power to extend and retract the actuator and blade.
Various blade nose configurations are used depending on the type of material being used and the sealing properties desired. For example, in liquid, gas and slurry applications, the nose of the blade typically seats against a resilient seal, such as an o-ring. In dry material applications, the nose edge of the blade is typically thrust upward against an elastomeric seal as an actuating cylinder pushes the blade horizontally. This combination of horizontal and vertical thrust motion can, however cause uneven seal wear as the leading edge of the blade has a higher seal seat contact pressure than the rear of the blade. It also stresses the cylinder shaft and places uneven loads on the cylinder seals. The perpendicular thrusting motion (relative to the direction of movement) places a torsional force on the actuator shaft causing premature wear of the actuator seals and potentially damaging the actuating cylinder. Any time a valve is operated in a pressurized environment the issues surrounding proper sealing are greatly enhanced.
Gate valves according to the prior art have most often been designed to be used with liquids and gases. These materials displace themselves when subject to the action of the gate and accordingly proper sealing of the valve is relatively easy to accomplish. “Bull nose” knife or gate valves have long been successfully used to control the flow of liquids and gases. Using gate valves with dry material presents problems with material “packing” into the valve seat area and getting trapped, preventing the valve from fully closing and seating. Particularly vulnerable is the void area formed between the valve housing and the radius of the typical rounded “bull nose” end of the blade. Material is easily trapped and wedged into the void formed between the blade and the housing. This trapped material may preclude proper sealing of the valve by preventing it from fully closing and will increase power consumption by the actuator as it works to overcome the obstruction. The “packed” material being handled may also be damaged by the wedging action. Trapped material also contributes to cross-contamination when different materials are routed thru the same valve.
At the rear edge of the blade, a sealed bonnet may be used to separate the actuating cylinder from the material path. This bonnet protects the working mechanism and also precludes materials passing through the valve from escaping into the environment. A bonnet seal assembly may be utilized to seal the bonnet area from the material flow opening. This bonnet seal is also subject to uneven wear and premature failure. The uneven perpendicular thrusting motion of the blade, referred to above, causes uneven compression on the bonnet seal, which in-turn leads to wear and premature seal failure. Since the bonnet seal provides the only isolation between the valve blade and the atmosphere, failure of the seal allows material to escape into the atmosphere. Material leaking into the bonnet may also interfere with proper valve operation.
Thus, there remains a need in the art for a positively sealing gate valve capable of handling dry bulk solid materials without the problems of seal failure and material packing inherent in the prior art valve designs.